This research project focuses on study of the first enzyme in platelet prostaglandin synthesis, prostaglandin synthetase. This enzyme has been purified to homogeneity and characterized by molecular weight, amino acid composition, and amino terminal sequence. The enzyme is inactivated by acetyl salicyclic acid by a mechanism of acetylation. The amino acid residue acetylated by aspirin has been identified as an amino terminus indicating that this residue is involved in the active site of the enzyme. Coupled with earlier data, it appears that prostaglandin synthetase is a dimer of nonidentical subunits, a finding which has implications for the regulation of enzyme activity. Currently, work is proceeding on a radioimmunoassay of prostaglandin synthetase in order to measure the enzyme in different disorders of platelet function. Furthermore, the enzyme is now being prepared in a purified active form in order to investigate the hypothesis that it consists of a dimer of two non-identical subunits.